We request funds for the purchase of an isotope ratio mass spectrometer for 18O and 13C analysis. We have selected the Thermoelectron Delta Plus equipped with a Gas Bench II for automated 18O analysis in CO2and a Combustion Elemental Analyzer for 13C for automated analysis of 13C in solids. This specific instrument provides fast analysis of 18O, while maintaining a high precision of 0.15 permil. This instrument, along with a current instrument used for 2H analysis, provides the optimal system for the analysis of energy expenditure by the doubly labeled water method. Our laboratory is one of the leading laboratories in the world for doubly labeled water analysis and has worked with dozens of users during the past two decades. We cannot, however, currently meet the requests from users due the age related down-time of our current 18O isotope ratio mass spectrometer and the absence of the automated inlet. This new instrument will dramatically improve increase our capacity to perform these isotope analyses and reduce our costs for the analyses. This new instrument will be housed in the Stable Isotope Laboratory of Nutritional Sciences at the University of Wisconsin. It will be operated under the direction Dr. Dale A. Schoeller and the operation of Timothy Shriver, MS. Oversight of the operation will provided by an executive committee composed of Drs. Colbert, Kemnitz and Schoeller to ensure equitable allotment of use among the users. The users group will consist of seven investigators. Four of these are from the University of Wisconsin and three from other US institutions. Most of the users will be utilizing the 18O analysis as it applies to doubly labeled water for the study of the etiology and treatment of obesity. Obesity remains the most prevalent and fastest growing health concerns in the United States. Obesity is among one of the top priorities on the NIH roadmap and the doubly labeled water method has been identified as one of the key research tools for the study o human obesity. In addition, this new instrument will provide new capacity in the form of the solids combustion inlet. This expands our potential users group in the area of development of dietary biomarkers. [unreadable] [unreadable]